The present invention is related to telecommunications antennas and, in particular, to alignment of microwave reflector antennas.
Microwave antennas for line-of-sight terrestrial communications and earth station satellite communications often use large curved reflectors and are sometimes referred to as xe2x80x9cdishxe2x80x9d antennas. The reflector comprises a metal material to reflect radio waves. For receiving signals, the large reflector is used to collect and focus electromagnetic waves into a receiver to obtain a stronger signal at the receiver. For transmitting, this type of antenna has a highly directional characteristic, resulting in efficient transmission of signals to a distant target.
FIG. 1 shows a typical antenna 100 comprising a main reflector 110. Antenna 100 depicted in FIG. 1 uses a particular arrangement of reflectors known as Gregorian optics. A typical commercially available antenna of this type is the 3.7 -meter earth station antenna which is available from Andrew Corporation 10500 W. 153rd St. Orland Park, Ill. 60462. In accordance with this arrangement, a subreflector 120 is suspended in front of the main reflector 110 by several struts 122 running from the main reflector 110 to subreflector 120. Subreflector 120 is substantially aligned with the axis of symmetry of main reflector 110. Protruding from main reflector 110 along the axis of symmetry and into the focal point of the reflector assembly is the so-called xe2x80x9cfeed hornxe2x80x9d 130. Feed horn 130 is a hollow waveguide through which radio signals at the antenna are coupled to electronic instruments such as receivers and transmitters. Feed horn 130 may conduct signals from a microwave transmitter coupled to antenna 100. The signals from the transmitter (not shown) are emitted from feed horn 130, strike subreflector 120, and then are reflected back to main reflector 110. From there, the signals are sent forth from antenna 100 to reach a distant target. Conversely, for receiving, the signals from a remote target strike main reflector 110, are collected and focused upon subreflector 120, the curvature of which causes the signals to become somewhat more focused and to be coupled into antenna feed horn 130.
Alignment of such an antenna is important to its performance because of the high degree of directionality of the antenna and the distances typically traversed by a radio signal, which may range from 10-30 miles in the case of terrestrial links to around 22,300 miles in the case of a satellite in geosynchronous orbit. At these distances, even a slight angular misalignment can cause loss of signal path. Misalignment or distortion in the shape of the antenna can also cause both received and transmitted signals to be weakened. At microwave frequencies, even slight distortions in the antenna or the waveguides used to coupled signals to and from the antenna can seriously affect signal quality. Sufficient distortions in the shape of the antenna can cause more complex forms of signal impairments, affecting frequency response and phase relationships.
Another aspect of antenna alignment relates to the polarization of the signal, referring to the orientation of the electric and magnetic components of the signal as it propagates. In order to successfully transmit a signal from one antenna to another, the receiver must be receptive to the same polarization emitted by the transmitter. Otherwise, even though the transmitting antenna may be directed to transmit signals at the receiving antenna, the signal may not be received if the transmitted signal is polarized in a substantially vertical direction while the receiver is receptive to signals that are polarized in a relatively horizontal direction. A corresponding receiver and transmitter must be aligned in terms of polarization.
Generally, adjustments are provided in the mounting of the antenna to allow coarse and fine positioning of the entire antenna assembly to point at a desired target. Furthermore, the orientation of the polarization for received or transmitted electromagnetic waves may be altered by rotating the feed horn, along with the associated waveguide couplers and such attached to the back end of the feed horn. Generally a feed horn assembly can be rotated within a reflector assembly. In one common arrangement, the base of the feed horn, the xe2x80x9cfeed hubxe2x80x9d is circular and concentrically nested into the so-called xe2x80x9cvertex openingxe2x80x9d at the center of the reflector. Once the feed horn assembly has been rotated to an optimum position, clamping fasteners around the perimeter of the feed hub are tightened to secure the feed horn assembly to the reflector.
For example, FIG. 1 shows a mounting ring 112 attached to reflector 110 such that mounting ring 112 surrounds the vertex opening at the back of reflector 110. Feed hub 132 is concentric with, and seated within, mounting ring 112. Feed hub 132 rigidly supports feed horn 130 and rotation of feed hub 132 accomplishes rotation of feed horn 130. Feed hub 132 comprises a number of fastener positions 136 and 137 where clamping fasteners may be placed to secure feed hub 132 in place within mounting ring 112.
In attempting to rotate the feed horn during polarization alignment, service personnel have difficulty finding a suitable place to apply torque to rotate the feed hub because feed hub 132 tends to bind with mounting ring 112. In many cases, tubular extension 134 is provided that protrudes behind the feed hub and is concentric with the feed tube. The manufacture of antenna intended that tubular extension of the antenna, be gripped by operational personal and used to rotate feed horn 130. However, tubular extension 134 is usually so short that for only one hand to adequately engage the tube. Furthermore, tubular extension 134 is of small diameter, increasing the difficulty with which service personnel can grip and apply torque to precisely rotate the feed horn assembly.
Often, in lieu of using this torque tube extension, service personnel will use other attachments to feed horn 130 to more easily apply torque. Although not shown in FIG. 1, waveguide couplers/combiners and electronic units, such as receiver front-ends, may be attached behind extension 134. When antenna 100 is configured with waveguide couplers/combiners and electronic units, feed hub 132 tends to bind even further with mounting ring 112, due to the cantilever affect from the weight of to waveguide and electronic units on the interfacing surfaces of feed hub 132 and mounting ring 112. The waveguide and/or electronics equipment presents a technician with a more prominent handhold for torqueing the feed tube assembly. However, applying torque to these sensitive components is risky and can result in the distorting or inadvertent breaking of waveguide feed components, potentially degrading or interrupting communications. Such damage can be extremely costly to repair. Moreover, some types of damage may be subtle enough to cause latent problems which are difficult and expensive to troubleshoot at a later time.
Thus, there is a need for an improved method by which a technician may easily and precisely align the polarization of the microwave antenna without exerting forces upon sensitive parts of the feed system.
The present invention provides for an improved method by which a feed system may be rotated within an antenna to facilitate the adjustment of polarization. The present invention provides for a novel polarization adjustment tool that engages the feed hub nested within the main reflector mounting ring. A technician using such a device may exert a torque directly to the feed hub without applying forces to other more delicate parts of the feed system.
In accordance with an exemplary embodiment, long lever arm is provided by the tool so that a technician may apply torqueing forces with less effort applied at the handle. The long lever arm improves the accessibility of the adjustment and dramatically improves the precision with which the feed assembly may be manually adjusted. The tool provided by the present invention offers these advantages over the prior art practice of using only a one-handed grip applied to the small torque tube behind the feed hub.
In accordance with an exemplary method of use, some of the points along the feed hub where fasteners are used to claim the feed hub to the mounting ring are also used as points where the adjustment tool engages the feed hub. In accordance with an exemplary embodiment of the present invention, the tool comprises at least two holes through which fasteners may be applied to attach the tool to the feed hub using the existing fasteners. Therefore, in accordance with an exemplary embodiment of the present invention, the holes in the tool are positioned so as to align with the positions of fasteners on the feed hub. In this manner, existing bolt holes or studs may be used to simply mount the tool atop the existing clamping hardware.
In accordance with an exemplary embodiment, the polarization adjustment tool is bifurcated at one end in order to straddle the central portion of the feed hub and to engage pairs of fasteners that are not adjacent along the edge of the feed hub. As will be described later, this aspect is useful where clamping fasteners and non-clamping fasteners are interspersed on the feed hub.
In accordance with an exemplary embodiment, the polarization adjustment tool comprises at least one offset portion which provides for clearance behind the feed hub. The clearance ensures that the tool does not interfere with the mounting ring and fasteners and especially allows access so that clamping fasteners xe2x80x9cunderxe2x80x9d the tool are accessible for loosening and tightening during the adjustment procedure.